Hot working die steel for die-casting

ABSTRACT

The invention provides a hot-working die steel for die-casting obtainable by quenching a steel comprising, in terms of % by mass, C: 0.1 to 0.3%, Si: 0.1 to 1.5%, Mn: 0.3 to 2%, Cr: 6 to 12%, P: 0.05% or less, S: 0.01% or less, Mo: 1 to 3%, V: 0.5 to 1.5%, s-Al: 0.005 to 0.025%, N: 0.005 to 0.025%, and O: 0.005% or less, with the remainder being Fe and inevitable impurities, followed by tempering the steel at a temperature of 500° C. or lower.

FIELD OF THE INVENTION

The present invention relates to a hot-working die steel for use asdie-casting molds. More particularly, the invention relates to ahot-working die steel for die-casting which inhibits the cracking from awater-cooling hole, which is a major cause of serious cracks indie-casting molds, and is capable of coping with a higher cycle speed inthe production of die-casting products. The hot-working die steel fordie-casting of the invention can be advantageously used as a materialfor aluminum die-casting molds.

BACKGROUND OF THE INVENTION

Aluminum die-casting molds have hitherto had a problem that cracksgenerate at the cavity surface due to thermal fatigue (i.e., heatcheck). This heat check is a phenomenon in which the cavity surface,when sprinkled with cooling water after mold opening, comes to have antensile stress due to a temperature difference between the rapidlycooled cavity surface and inner parts in a heated state, and the thermalfatigue resulting from repetitions of this stress generation causescracks at the cavity surface.

It is said that it is advantageous to heighten the hardness of the moldfor diminishing the heat check.

On the other hand, there recently has been a desire for a reduction incycle time (higher cycle speed) in the production of aluminumdie-casting products. For the purpose of reducing a mold closing time inorder to realize that desire, the water cooling of an aluminum cast in amold tends to be enhanced. Specifically, this enhancement of watercooling is accomplished by disposing a water-cooling hole in a positioncloser to the cavity surface. In this case, the thermal stressgenerating at the surface of the water-cooling hole during the castingof an aluminum product is increased and the phenomenon in which a crackgenerates from the water-cooling hole becomes problematic.

Such a crack generating from a water-cooling hole is not attributableonly to the thermal stress repeatedly imposed during casting but isthought to be a delayed-fracture phenomenon including a combination ofcracking caused by thermal stress and stress corrosion cracking causedby rust generating on the surface of the water-cooling hole.

The higher the hardness of a mold, the more the cracking from thewater-cooling hole is apt to occur. Consequently, it is advantageous toreduce the hardness of a mold for inhibiting such cracking from thewater-cooling hole.

Namely, to increase mold hardness is advantageous for diminishing theheat check but is disadvantageous for diminishing cracking from awater-cooling hole, whereas to reduce mold hardness is advantageous fordiminishing cracking from a water-cooling hole but is disadvantageousfor diminishing the heat check, resulting in impaired heat checkresistance.

From the standpoint of inhibiting the cracking from a water-coolinghole, it is desirable to regulate the mold hardness to HRC 45 to 40.

Hot-working die steels of the 5Cr type represented by JIS-SKD61 havebeen mainly used for current aluminum die-casting molds. In recentyears, the use hardness thereof has been increasing so as to inhibit theheat check generating at the cavity surface, and the risk of crackingfrom the water-cooling hole in the mold has been increasing with thetrend toward a higher cycle speed in the production of aluminumdie-casting products.

In the case of the JIS-SKD61, this steel contains about 0.4% of C andthe hardness of the steel in a quenched state is, for example, about HRC53.

For reducing the hardness thereof to HRC 45 or lower for the purpose ofinhibiting cracking from a water-cooling hole, it is necessary toconduct annealing at a high temperature of 600° C. or above. However,when annealing at such a high temperature is conducted, the corrosionresistance of the steel decreases considerably.

This material, which contains Cr in an amount of about 5%, in itself isa material having excellent corrosion resistance. However, when thissteel is annealed at a temperature as high as 600° C. or above, most ofthe Cr contained therein separates out as a Cr carbide due to thishigh-temperature annealing. Accordingly, the Cr contained in the steelthus comes not to contribute to an improvement in corrosion resistance.

In any event, the hot-working die steels presently in main use asaluminum die-casting molds, which are represented by JIS-SKD61, areineffective in satisfactorily overcoming the problem concerning crackingfrom a water-cooling hole.

It is thought that an effective measure in satisfactorily overcomingeach of the problem concerning cracking from a water-cooling hole andthe problem concerning heat check at the cavity surface is to preventrusting in the water-cooling hole and to reduce the hardness of thatinner part of the mold in which the water-cooling hole is present, aswell as to increase the hardness of the mold cavity surface where a heatcheck may generate. However, no material satisfying such properties hasbeen provided yet.

Incidentally, reference document 1 shown below discloses an inventionconcerning a technique in which the inner circumferential surface of thewater-cooling hole of a die-casting mold is regulated so as to have alower hardness than the mold surface to thereby reconcile the preventionof water-cooling hole cracking and the heat check resistance of the moldsurface.

The steel disclosed in this reference document 1 is produced byregulating JIS-SKD61, which has been used hitherto, so as to have a highhardness by quenching and tempering and then regulating the surface ofthe water-cooling hole so as to have a low hardness by local temperingwith induction heating, burner heating, laser heating, or the like.

All the methods disclosed in this reference document 1 necessitate localheating, and have a problem that the shape of the water-cooling hole islimited, for example, that the diameter of the water-cooling hole shouldbe a size which enables burner insertion.

Reference Document 1: JP-A-6-315753

SUMMARY OF THE INVENTION

The present invention has been achieved under the circumstancesdescribed above. An object of the invention is to provide a hot-workingdie steel for die-casting which has excellent heat check resistance andcan satisfactorily inhibit cracking from a water-cooling hole.

The present inventors have made eager investigation to examine theproblem. As a result, it has been found that the foregoing objects canbe achieved by the following hot-working die steels for die-casting.With this finding, the present invention is accomplished.

The present invention is mainly directed to the following items.

1. A hot-working die steel for die-casting obtainable by quenching asteel comprising, in terms of % by mass,

C: 0.1 to 0.3%,

Si: 0.1 to 1.5%,

Mn: 0.3 to 2%,

Cr: 6 to 12%,

P: 0.05% or less,

S: 0.01% or less,

Mo: 1 to 3%,

V: 0.5 to 1.5%,

s-Al: 0.005 to 0.025%,

N: 0.005 to 0.025%, and

O: 0.005% or less,

with the remainder being Fe and

inevitable impurities,

followed by tempering the steel at a temperature of 500° C. or lower.

2. The hot-working die steel for die-casting according to item 1, whichfurther comprises at least one member selected from the group consistingof, in terms of % by mass,

Ni: 2% or less, and

Cu: 1% or less.

3. The hot-working die steel for die-casting according to item 1 or 2,which further comprises, in terms of % by mass,

C: 5% or less.

4. The hot-working die steel for die-casting according to any one ofitems 1 to 3, which further comprises at least one member selected fromthe group consisting of, in terms of % by mass,

Ti: 0.2% or less,

Zr: 0.2% or less, and

Nb: 0.2% or less.

The hot-working die steel for die-casting of the invention has a reducedC content and, on the other hand, has high and optimized Cr and Mocontents. Accordingly, the steel of the invention, when used as adie-casting mold, can effectively inhibit cracking from thewater-cooling hole and can impart excellent heat check resistance to thedie-casting mold. The hot-working die steel for die-casting of theinvention can be advantageously used especially as a material foraluminum die-casting molds.

Cr is known as an element which improves corrosion resistance. Inordinary JIS-SKD61, however, the Cr for improving corrosion resistanceseparates out disadvantageously as a carbide during the heat treatmentfor obtaining a use hardness because this steel is tempered at atemperature as high as 600° C. or above as described hereinabove.Accordingly, the effect of the Cr is almost lost. On the other hand,when the tempering temperature is lowered to such a degree that Crcarbide separation does not occur, the steel comes to have anexceedingly high hardness of 50 HRC or above. When such a steel is usedas a die-casting mold, cracking from the water-cooling hole is apt tooccur.

A target hardness may be obtained through tempering at a low temperatureof 500° C. or below by reducing the C content. In this case, however,the hardness of the cavity surface also decreases to cause a problemthat heat check resistance becomes poor.

Herein, in the hot-working die steel for die-casting of the invention,the C content is reduced and Mo is added in an appropriate amount.

By reducing the C content, a hardness of HRC 45 or below, which is lessapt to result in cracking from a water-cooling hole, can be obtainedthrough tempering at a low temperature of 500° C. or below.

Furthermore, by the addition of an appropriate amount of Mo, the moldcavity surface can be partly increased in hardness by utilizing the heattransferred from the melt (e.g., aluminum melt) during die-casting whenthis steel is used as a die-casting mold.

Specifically, the Mo added separates out as a carbide when the mold isused for the casing of a die-casting product and the cavity surface isheated by the heat transferred from the melt (about 600-650° C. in thecase of aluminum melt) to thereby serve to partly heighten the hardnessof the cavity surface.

Namely, the hot-working die steel for die-casting of the invention hasan effect that the hardness of the cavity surface increases by means ofage hardening during the use of the mold. Due to this effect, heat checkin the cavity surface can be satisfactorily inhibited.

Namely, in the hot-working die steel for die-casting of the invention,the phenomenon in which, when the steel is used as a die-casting mold,the cavity surface thereof undergoes age hardening due to the heattransferred from the melt can be ingeniously utilized. As a result, itis possible to obtain a mold which retains a low hardness in inner partsthereof but has a partly increased hardness in the cavity surface. Inthis respect, the hot-working die steel for die-casting of the inventionhas an excellent effect over conventional ones.

Moreover, Cr as a corrosion-resistant element has been added in a largeramount in the invention than in JIS-SKD61. In the invention, annealingis conducted at a temperature as low as 500° C. or below after aquenching treatment. Accordingly, the Cr added does not separate out asa carbide but is in the state of being a solid solution in the matrix toeffectively serve to improve the corrosion resistance of the steel.Namely, due to this corrosion-resistance-improving function of the Cr,when the hot-working die steel for die-casting of the invention is usedas a die-casting mold, rusting in the water-cooling hole is inhibitedand the cracking from the water-cooling hole, which is caused by therusting, is satisfactorily inhibited.

Furthermore, when the hot-working die steel for die-casting of theinvention is used as a die-casting mold, the cavity surface of the moldundergoes secondary hardening (age hardening) due to the separation of aMo carbide, whereby it hardens to come to have a hardness of HRC 45 orhigher, at which heat check resistance can be secured.

Next, reasons for the limitation of each chemical component in theinvention will be described below in detail. Hereinafter, “%” means “%by mass”.

C: 0.1 to 0.3%

C is an element necessary for securing hardness and wearing resistance,which are important mold performances.

Ordinary hot-working die steels contain C in an amount of about 0.4%. Inthe invention, however, the C content is lower than in the ordinaryhot-working die steels so that a hardness of HRC 45 or lower can beobtained through low-temperature tempering at 500° C. or lower. Therange thereof is 0.1 to 0.3%, preferably 0.15 to 0.25%.

Si: 0.1 to 1.5%

Si is an element necessary as a deoxidizing element in steelmaking.

Furthermore, by increasing the content thereof, machinability andresistivity to temper softening can be improved.

However, excessively large addition amount thereof results in reducedimpact value toughness. Consequently, the range of the addition amountthereof is 0.1 to 1.5%, preferably 0.1 to 0.5%.

Mn: 0.3 to 2%

Mn is a component necessary for securing hardenability and hardness. Theaddition amount thereof id set at 0.3% or larger.

On the other hand, when Mn is added excessively, hardenability becomestoo high and there are some cases where quenching yields a large amountof residual γ to reduce the impact value or where annealing does notresult in a reduction in hardness. Consequently, the upper limit thereofis set at 2%. The upper limit of the addition amount of Mn is preferablyset at 1%.

Cr: 6 to 12%

Cr is an element which improves hardenability and also improves thecorrosion resistance of a water-cooling hole.

For obtaining the effect of improving corrosion resistance, it isnecessary to add Cr in an amount of 6% or larger. It is preferred to addCr in an amount of 8% or larger.

However, addition in an excessively large amount reduces resistivity totemper softening and also reduces mold performances. Therefore, theupper limit thereof is set at 12%. Further, it is preferred that theupper limit of the content of Cr be set at 10%.

P: ≦0.05%

P is an element which is preferably diminished because it reduces impactvalue. When the steel contains it inevitably, it is preferred todiminish the content thereof to 0.05% or below.

S: ≦0.01%

S is an element which is preferably diminished because it forms MnS toreduce impact value.

When the steel contains it inevitably, it is preferred to diminish thecontent thereof to 0.01% or below.

Mo: 1 to 3%

Mo is necessary for strengthening the matrix and improving the wearingresistance through carbide formation and also for securinghardenability.

Furthermore, when the hot-working die steel for die-casting of theinvention is used as a die-casting mold, this Mo carbide separates outdue to the heat transferred from the melt (around 600° C. in the case ofaluminum melt) to thereby heighten the hardness of the mold.

Although the mold hardness after quenching and subsequent tempering hasbeen set at HRC 45 or lower in the invention in order to preventcracking from the water-cooling hole, the temperature of the cavitysurface rises during die-casting (around 600° C. in the case of aluminumdie-casting) and a hardness of HRC 45 or higher can be obtained. Thus,heat check resistance can be improved.

For obtaining such an effect, it is necessary to add Mo in an amount of1% or larger and it is preferred to add Mo in an amount of 1.5% orlarger.

However, even when it is added excessively, the effect is saturated andsuch an excessive addition is economically disadvantageous. The upperlimit of the addition is therefore set at 3%. It is preferred that theupper limit of the addition amount of Mo be set at 2.5%.

V: 0.5 to 1.5%

V is an element which forms a carbide and separates out during temperingto thereby strengthen the matrix and improve wearing resistance.

Furthermore, during heating for quenching, it forms a fine carbide andthis has the effect of inhibiting crystal grain enlargement to therebyinhibit impact value decrease.

For obtaining such an effect, it is necessary to add V in an amount of0.5% or larger.

On the other hand, in a case where V is added excessively, it yieldscoarse carbonitride crystals during solidification to reduce toughness.Consequently, the upper limit of the addition amount of V is set at1.5%. It is preferred that the upper limit of the addition amount of Vbe set at 1%.

s-Al: 0.005 to 0.025%

Al not only functions as a deoxidizing element during steelmaking, butis an element which combines with the N in the steel and finelydisperses as a nitride to inhibit crystal grain enlargement duringheating for quenching.

For obtaining such effects, it is necessary to add Al in an amount of0.005% or larger.

However, even when it is added in a large amount, the effect issaturated.

Consequently, the upper limit of the addition amount thereof is set at0.025%.

N: 0.005 to 0.025%

N is an element which combines with the Al and V in the steel to formnitrides. The nitrides finely disperse to thereby inhibit crystal grainenlargement during heating for quenching. N is hence an elementeffective for preventing impact value decrease.

For obtaining such an effect, it is necessary to add N in an amount of0.005% or larger.

However, even when it is added in a large amount, the effect issaturated.

Consequently, the upper limit of the addition amount thereof is set at0.025%.

O: ≦0.005%

O forms oxide inclusions to decrease impact value. For inhibiting impactvalue decrease, it is necessary to reduce the content of O to 0.005% orlower.

Ni: ≦2%, Cu: ≦1%

Since Ni enhances hardenability and are thus effective in toughening thematrix, it can be added according to need.

However, even when these elements are added excessively, the effects aresaturated and the excessive addition thereof is economicallydisadvantageous. The upper limits of the addition amount thereof arehence set at 2% and 1%, respectively.

Co: ≦5%

Co is an element which improves strength through solid-solutionstrengthening. It can be added according to need.

However, even when it is added excessively, the effect is saturated andthe excessive addition thereof is economically disadvantageous.Consequently, the upper limit of the addition amount thereof is set at5%.

Ti: ≦0.2%, Zr: ≦0.2%, Nb: ≦0.2%

These are elements which form Ti(CN), Zr(CN), Nb(CN), and compositecarbonitrides thereof and finely separate out to inhibit crystal grainenlargement during heating for quenching. When it is desired to formfine crystal grains to secure toughness, these elements can be addedaccording to need.

However, in case where those elements are added excessively, theyseparate out as coarse carbonitride crystals during solidification toreduce rather than increase impact value. Consequently, the upper limitsof the addition amount thereof are set at 0.2%, respectively.

Furthermore, in the case where those elements are added in combination,it is preferred that the total amount thereof be 0.5% or smaller.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will be described below in detail.

The present invention is now illustrated in greater detail withreference to Steels of the invention and Comparative Steels, but itshould be understood that the present invention is not to be construedas being limited thereto.

Steels respectively having the compositions shown in Table 1 each weremelted in a 150-kg vacuum high-frequency induction furnace. Each ingotthus obtained was forged at 1,200° C. into a square bar having a sectionof 60 mm×60 mm.

This square bar was cut into a length of 500 mm, subsequently heated to1,030° C., and then subjected to oil quenching.

Thereafter, tempering was conducted twice under the conditions with atemperature of 450° C. and a period of 1 h. Each square bar which hadbeen tempered was subjected to each of a measurement of the hardness ofa ¼ H part (a part located midway between the surface and the centralpart), a Charpy impact test in the T direction (width direction for thesquare bar) using a 2-mm U-notch test piece, and a corrosion test inwhich a block of 10 mm×10 mm×10 mm was cut out of the ¼ H part, thesurface thereof was polished with an emery paper, and this block wasthen wholly immersed in 20° C. industrial water for 24 h and examinedfor rusting.

In the evaluation of corrosion resistance, ones which suffered norusting are rated as A and ones which suffered rusting are rated as B.

Furthermore, for the purpose of simulating a heat history in repetitionsof the casting of an aluminum die-casting product, each of the squarebars which had been tempered at 450° C. was subjected to repeated 1,000cycles each including heating from room temperature to 650° C. byhigh-frequency heating, holding at this temperature for 4 seconds, andsubsequent water cooling. Thereafter, the surface hardness thereof wasmeasured.

The results of these evaluations are shown in Table 2.

TABLE 1 Composition (mass %) No. C Si Mn P S Cr Mo V s-Al N O OthersInvention 1 0.12 0.1 0.42 0.008 0.002 8.2 2.8 0.6 0.015 0.01 0.003 Steel2 0.2 0.3 0.45 0.012 0.002 9 2.5 0.8 0.02 0.012 0.002 3 0.18 0.5 0.80.01 0.005 11.3 2 1.1 0.005 0.022 0.004 4 0.23 0.3 1.2 0.024 0.008 6.52.8 1.4 0.008 0.008 0.002 5 0.28 0.8 0.6 0.003 0.009 10.1 1.6 0.9 0.0130.005 0.003 6 0.2 1.3 1.4 0.018 0.001 8.8 1.8 0.8 0.022 0.008 0.001 Ni:1% 7 0.2 0.3 0.5 0.009 0.002 9.2 2.3 0.6 0.021 0.011 0.002 Ni: 0.5%, Cu:0.5% 8 0.21 0.25 0.45 0.003 0.002 9 2.5 0.7 0.018 0.009 0.003 Ni: 0.7%,Co: 2% 9 0.18 0.3 0.6 0.007 0.002 9.5 2.2 0.6 0.011 0.021 0.002 Co: 4%,Ti: 0.05% 10 0.22 0.22 0.65 0.031 0.001 10.1 2.3 0.55 0.016 0.023 0.002Zr: 0.1%, Nb: 0.1% 11 0.18 0.25 0.67 0.021 0.001 8.9 2.5 0.61 0.02 0.0180.003 Co: 1%, Zr: 0.2%, Nb: 0.05% Comparative a 0.05 0.2 0.52 0.0150.002 8.3 2.3 0.62 0.021 0.011 0.002 Steel b 0.38 0.21 0.48 0.011 0.0029.1 2.3 0.65 0.022 0.015 0.002 c 0.2 2 0.45 0.012 0.002 9.2 2.5 0.60.018 0.012 0.002 d 0.2 0.2 2.5 0.011 0.002 8.9 2.4 0.61 0.019 0.0110.003 e 0.2 0.2 0.5 0.08 0.001 9.1 2.5 0.6 0.021 0.011 0.002 f 0.2 0.210.5 0.012 0.05 9 2.5 0.61 0.02 0.01 0.002 g 0.21 0.22 0.5 0.011 0.0015.1 2.3 0.6 0.021 0.01 0.002 h 0.19 0.21 0.45 0.012 0.002 13.5 2.4 0.610.019 0.009 0.003 i 0.21 0.25 0.44 0.009 0.002 9 0.6 0.6 0.02 0.0090.002 j 0.2 0.21 0.48 0.011 0.002 9.1 2.1 0.3 0.015 0.008 0.001 k 0.190.22 0.51 0.011 0.002 9.3 2.4 0.6 0.003 0.007 0.002 l 0.21 0.3 0.520.012 0.001 9 2.3 0.7 0.012 0.002 0.002 m 0.19 0.29 0.46 0.011 0.001 92.3 0.6 0.012 0.009 0.008 Conventional A 0.38 1 0.45 0.011 0.001 5.5 1.20.85 0.02 0.012 0.002 Steel

TABLE 2 450° Tempering Hardness after Impact repetitions Hardness valueCorrosion of 650° No. (HRC) (J/cm²) resistance heating (HRC) Invention 140 52 A 46 Steel 2 42 48 A 48 3 41 50 A 46 4 43 46 A 49 5 44 45 A 48 642 48 A 47 7 42 48 A 48 8 42 49 A 47 9 41 50 A 46 10 43 48 A 48 11 41 48A 47 Com- a 36 58 A 42 parative b 53 21 A 48 Steel c 42 25 A 48 d 42 23A 48 e 42 18 A 47 f 42 15 A 48 g 42 49 B 48 h 42 21 A 48 i 42 48 A 44 j42 32 A 47 k 41 33 A 47 l 42 28 A 48 m 40 30 A 48 Conventional A 53 18 B47 Steel * Corrosion resistance: A . . . no rusting, B . . . rustingoccurred

Furthermore, a steel obtained by heating Invention Steel No. 2 shown inTable 1 to 1,030° C. and subsequently subjecting it to oil quenching andthen to tempering twice under the conditions with a temperature of 450°C. and a period of 1 h, one obtained by heating Conventional Steel A to1,030° C. and subsequently subjecting it to oil quenching and then totempering twice under the conditions with a temperature of 450° C. and aperiod of 1 h, and one obtained by subjecting Conventional Steel A totempering twice under the conditions with a temperature of 630° C. and aperiod of 1 h were respectively evaluated for delayed-fractureresistance as an index to receptivity to cracking from a water-coolinghole.

Here, the evaluation of delayed-fracture resistance was conducted in thefollowing manner.

Namely, industrial water was dropped (in order to cause rusting) ontothe notched part of a test piece having a 0.1-R annular notch, and therelationship between flexural stress and fracture time was examined.

The delayed-fracture resistance was evaluated by comparing in the ratioof static flexural stress (0-h rupture stress) to the stress causingrupture at 200 h.

Furthermore, 10,000 cycles each including heating from room temperatureto 650° C., holding at this temperature for 4 seconds, and subsequentwater cooling were repeatedly conducted. Thereafter, the length of theheat crack generated at the surface was measured and evaluated as anindex to heat check resistance.

The results of these evaluations are shown in Table 3.

In Table 3, the desired value of delayed-fracture resistance was set at0.7 or higher.

TABLE 3 Heat check Delayed-fracture Tempering resistance resistancetemperature Hardness Length of largest Proportion of 200-h No. (° C.)(HRC) heat crack (μm) rupture stress 2 450 42 120 0.98 A 450 53 123 0.65630 42 253 0.91

As the results given in Table 2 show, Invention Steels No. 1 to No. 11have hardnesses of HRC 40 to 44 after the tempering at 450° C. and havehardnesses after the repetitions of heating at 650° C. of HRC 46 to 49.The hardnesses thereof have increased.

Furthermore, since the tempering is low-temperature tempering at 450°C., almost no Cr carbide has separated out. Each steel showssatisfactory corrosion resistance.

In contrast, Comparative Steel a has a C content of 0.05%, which islower than the lower limit of 0.1% in the invention, and hence has ahardness after the 450° C. tempering as low as HRC 36. The hardnessthereof after the repetitions of heating at 650° C. also is as low asHRC 42. It has poor heat check resistance.

Comparative Steel b conversely has a C content of 0.38%, which is higherthan the upper limit of 0.3% in the invention, and hence has a hardnessafter the 450° C. tempering as high as HRC 53. It has a low impactvalue.

Comparative Steel c has a Si content of 2%, which is higher than theupper limit of 1.5% in the invention. It has a low impact value.

Comparative Steel d has a Mn content of 2.5%, which is higher than theupper limit of 2% in the invention. It has a low impact value.

Comparative Steel e has a content of P as an impurity of 0.08%, which ishigher than the upper limit of 0.05% in the invention. This steel alsohas a low impact value.

Furthermore, Comparative Steel f has a content of S also as an impurityof 0.05%, which is higher than the upper limit of 0.01% in theinvention, and hence has a low impact value.

Comparative Steel g has a Cr content of 5.1%, which is lower than thelower limit of 6% in the invention, and hence has low corrosionresistance.

Comparative Steel h conversely has a Cr content of 13.5%, which ishigher than the upper limit of 12% in the invention, and hence has a lowimpact value.

Comparative Steel i has a Mo content of 0.6%, which is lower than thelower limit of 1% in the invention. Because of this, even through therepetitions of heating at 650° C., the hardness has not increasedsufficiently. This means that heat check resistance is insufficient.

Comparative Steel j has a V content of 0.3%, which is lower than thelower limit of 0.5% in the invention. Because of this, crystal grainenlargement has occurred and the steel has a low impact value.

Comparative Steel k has an s-Al content of 0.003%, which is lower thanthe lower limit of 0.005% in the invention. Because of this, crystalgrain enlargement has occurred and the steel has a low impact value.

Comparative Steel l has an N content of 0.002%, which is lower than thelower limit of 0.005% in the invention. Because of this, crystal grainenlargement has occurred in this case also and the steel has a lowimpact value.

Comparative Steel m has an O content of 0.008%, which is higher than theupper limit of 0.005% in the invention. Because of this, the steelcontains a larger amount of inclusions and has a low impact value.

Next, Conventional Steel A is JIS-SKD61 and has a hardness after the450° C. tempering of HRC 53. The hardness thereof after the repetitionsof heating at 650° C. has decreased to HRC 47. It is poor also incorrosion resistance.

Next, in Table 3, Invention Steel No. 2 has a low hardness after thelow-temperature tempering at 450° C. However, this steel is equal inheat check resistance and superior in delayed-fracture resistance to thehigh-hardness material obtained by tempering Conventional Steel A at450° C.

Furthermore, as compared with the steel having the same hardnessobtained by the 630° C. high-temperature tempering of Conventional SteelA, Invention Steel No. 2 has higher corrosion resistance and better heatcheck resistance because of the low-temperature tempering.

It can be seen as demonstrated above that the steels of the inventionhave both of the property of inhibiting cracking from a water-coolinghole and heat check resistance; these two properties have hitherto beinginconsistent with each other.

While the present invention has been described in detail and withreference to specific embodiments thereof, it will be apparent to oneskilled in the art that various changes and modifications can be madetherein without departing from the spirit and scope thereof.

The present application is based on Japanese Patent Application No.2005-346156 filed on Nov. 30, 2005, and the contents thereof areincorporated herein by reference.

1. A hot-working die steel for die-casting obtainable by quenching asteel comprising, in terms of % by mass, C: 0.1 to 0.3%, Si: 0.1 to1.5%, Mn: 0.3 to 2%, Cr: 6 to 12%, P: 0.05% or less, S: 0.01% or less,Mo: 1 to 3%, V: 0.5 to 1.5%, s-Al: 0.005 to 0.025%, N: 0.005 to 0.025%,and O: 0.005% or less, with the remainder being Fe and inevitableimpurities, followed by tempering the steel at a temperature of 500° C.or lower.
 2. The hot-working die steel for die-casting according toclaim 1, which further comprises at least one member selected from thegroup consisting of, in terms of % by mass, Ni: 2% or less, and Cu: 1%or less.
 3. The hot-working die steel for die-casting according to claim1, which further comprises, in terms of % by mass, Co: 5% or less. 4.The hot-working die steel for die-casting according to claim 2, whichfurther comprises, in terms of % by mass, Co: 5% or less.
 5. Thehot-working die steel for die-casting according to claim 1, whichfurther comprises at least one member selected from the group consistingof, in terms of % by mass, Ti: 0.2% or less, Zr: 0.2% or less, and Nb:0.2% or less.
 6. The hot-working die steel for die-casting according toclaim 2, which further comprises at least one member selected from thegroup consisting of, in terms of % by mass, Ti: 0.2% or less, Zr: 0.2%or less, and Nb: 0.2% or less.
 7. The hot-working die steel fordie-casting according to claim 3, which further comprises at least onemember selected from the group consisting of, in terms of % by mass, Ti:0.2% or less, Zr: 0.2% or less, and Nb: 0.2% or less.
 8. The hot-workingdie steel for die-casting according to claim 4, which further comprisesat least one member selected from the group consisting of, in terms of %by mass, Ti: 0.2% or less, Zr: 0.2% or less, and Nb: 0.2% or less.